This invention relates to compression methods and apparatus for seismic data.
Data compression (or reduction) is a digital signal processing technique for reducing the amount of data to be dealt with without losing essential information in the process. This is essentially done by the removal of redundancy in the data and may involve the discarding of uninteresting parts of the data. Such compression can result in some loss of data accuracy. Data compression that allows the exact reconstruction of the original data is often referred to in the literature as lossless. Data compression that involves some reduction in accuracy is known as lossy. Common examples of data compression are xe2x80x9croundingxe2x80x9d and xe2x80x9cdown samplingxe2x80x9d; both methods are usually lossy.
Seismic data acquisition requires a large number of seismic experiments to be conducted in order to obtain a reliable image of the Earth""s subsurface. Each experiment involves the generation of a sound wave using an appropriate source and measuring the earth""s response by a large number of receivers. A large scale seismic survey thus produces an enormous amount of data which will normally be in digital format, which has to be transmitted, stored and processed. To facilitate the handling of such large volumes of data, data compression can be utilized.
A lossy data compression technique that is routinely used in seismic data acquisition is group forming. This involves the retention and transmission and processing of the sum of neighboring receivers within fixed-sized groups, instead of the individual measurements.
Group forming is not used primarily for data compression. Group forming suppresses random ambient noise and suppresses waves with low apparent velocities, such as groundroll in land seismics. Thus group forming attenuates the high spatial frequency content of the data. However, the attenuation is performed in a crude way as it only partially suppresses apparently slowly propagating waves and alters the rest of the data. Consequently there is a good reason to omit group forming from the acquisition stage and to record the output of every receiver individually. This then permits the application of more sophisticated methods for reducing random and coherent noise. However the abolition of group forming at the acquisition stage greatly increases the amount of data to be handled downstream.
In IEEE Int. SYM. Circuits and Systems, New Orleans, La., May 1-3, 1990, Vol. 2, 1573-6, A. Spanias, S. Johnson et al. describe several transform based methods for seismic data compression. The methods include the Discrete Fourier Transformation (DFT), the Discrete Cosine Transformation (DCT), the Walsh-Hardamard Transform (WHT), and the Karhunen-Loeve Transform (KLT). However the DCT in the form described in the publication and applied to a sliding frame of N data points can be used for a relative comparison between several different transformations. When applied as data compression method, the sliding frame produces a large amount of redundant data in the transform domain.
It is therefore an object of the present invention to provide a method for compressing seismic data. It is another object of the invention to provide a method for compressing seismic data without using group forming.
The invention provides a first level of compression in which local spatial or temporal discrete trigonometric (i.e. either sine or cosine) transformations of type IV are applied to seismic data signals (FIG. 4, step 314). Discrete sine/cosine transformations of type IV are known as such. A general description is given for example by H. S. Malvar in: xe2x80x9cLapped transforms for efficient transform/subband codingxe2x80x9d, IEEE ASSP, vol. 38, no. 6, June 1990. The local spatial or temporal discrete sine/cosine transformation results in transform coefficients which are more compact and less correlated that the original data. Both of these properties can be advantageously exploited in subsequent data processing steps.
The compactness of the transform coefficient is exploited in a processing step, which can be described as a requantization or round-off step. The purpose of this step is to retain selected coefficients at high accuracy and other coefficients at lesser accuracy so as to reduce the quantity of data needed to describe the coefficients and thereby achieve further data compression.
The reduced correlation of the transform coefficients provides an opportunity to apply encoding schemes so as to further reduce the amount of data to be stored or transmitted. Applicable coding schemes are known as such, e.g., Huffmann coding or Amplitude coding.
The seismic data signals to which the method is applied are traces typically obtained from a number of receivers, e.g. geophones or hydrophones (FIG. 4, step 310). These may be arranged in combinations all of which are well known in the prior art. One of these is, for example, a conventional 3-D land seismic layout of linear arrays of geophones arranged in a number of parallel lines. The use of local transformations in the method permits the compression of the data over a certain numbers of receivers contained within each line. A local transformation is one in which the transformation is applied over defined windows of traces, as is well understood in the art. Thus the number of traces over which the local transformation is applied at each successive stage of the transformation is referred to as a spatial window and the window may be varied according to which type of transformation is applied.
The windows of the local transformations are defined by a window function, the window function being chosen so that the transformation is orthonormal and invertible. The window function is chosen so that the transformations are applied over a central window overlapping the adjoining windows (FIG. 4, step 312), most preferably overlapping half of those windows.
The transformation may be performed in two steps, the first step comprising a folding step in which the central window is combined with adjacent half windows to produce a folded signal and the second step comprising compressing a cosine transformation which is performed on the folded signal.
In addition to the local spatial transformation, preferably a local temporal transformation is applied to the data. The combination of both local transformations achieves a better compression ratio. The local temporal transformation is preferably a local temporal discrete sine/cosine transformation of type IV. However other signal transformations and decompositions may be used, such as an ordinary local discrete cosine transformation, and a local fourier transformation. The local spatial trigonometric transformation and local temporal trigonometric transformation may be applied in any order.
The transform coefficients, as representing the original data after the transformation, form a set of data to which different compression methods can be applied (FIG. 4, step 316). These compression methods may be collectively referred to as (re-) quantization (FIG. 5, step 332) and encoding (FIG. 5, step 340).
The quantization process when used for compressing data usually includes a scaling step and a round-off. The quantization process is designed to reduce the high-frequency components or coefficients while maintaining the low-frequency components with higher accuracy.
The scaling is preferably achieved through dividing by first scalar coefficients representing low frequencies and dividing by second scalar coefficients representing high frequencies. The first scalar is chosen to be less than the second scalar, since the larger the scalar, the greater the compression which will be achieved. In this way the coefficients representing the low frequencies which are of particular interest in seismic analysis will not be compressed so much as these representing high frequencies, so that the accuracy of the former is preserved.
The scaling may be achieved by uniform quantizing (FIG. 5, step 334) using a nearest integer function, or by statistical rounding, or non-uniform requantization (FIG. 5, step 336).
The scaling or quantization parameters may vary with time, space or spatial or temporal frequency.
In a further preferred embodiment of the invention, the compression ratio is automatically determined by the noise level in the seismic signal. The noise level is preferably measured using a part or parts of the signal which contain no signal generated by the seismic source. Thus, part of the traces recorded prior to the xe2x80x9cfirst arrivalxe2x80x9d or so-called noise records, i.e. traces recorded in the absence of a seismic source, can be utilized to determine the noise level. Even more preferably, the signal is filtered before the step of determining the noise level to avoid an overestimation of the noise level.
Preferably the compression ratio, i.e. the quantization error is chosen equal or lower than the noise level.
The reduction in accuracy caused by the compression of the data increases the redundancy in the data. Therefore according to a further feature of the invention, the data redundancy is used to further reduce the compressed data, preferably by applying variable-length data coding, such as Amplitude coding or Huffman coding. For example the scale coefficients may be amplitude encoded so that each coefficient becomes proportional to the absolute value of the largest coefficient.
The seismic data compression method in accordance with the invention can be applied to all types of seismic data, including 2D and 3D survey data of land, transition zone, marine or sea bottom acquisitions. Possible data also includes pre-recorded or pre-processed data, such as shot gathers, Common Midpoint (CMP) gathers, stacks, migrated sections or single sensor recordings. It is also suitable for use with a two dimensional seismic acquisition geometry, such as a land layout comprising one line of receiver, or for two- or three- dimensional marine seismic surveys using streamers and hydrophones, and application in two perpendicular directions to a true areal receiver layout (inline/crossline application).
Using dedicated chip sets, it is feasible to compress the data in a single receiver before transmission to field boxes or acquisition trucks or ships. It can also be used in data transmission further xe2x80x9cdown-streamxe2x80x9d in the data processing, e.g. for the transmission of data to a processing center or for intermediate storage. After transmitting and/or storing the compressed data (FIG. 4, step 318), the original data can be reconstructed or decompressed by applying the same steps inverted and in reverse order (FIG. 4, step 320).
The invention also lies in apparatus for performing the methods described herein.
The invention also lies in a seismic survey in which data is subjected to compression according one or more of the foregoing methods.
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the detailed description and drawings following hereinbelow.